The inventors have previously invented a silica film having particularly advantageous properties. The film has useful anti-fogging and anti-reflection properties, amongst other potentially useful properties. One method of production of one form of the silica film is described in our co-pending International Application number WO 2005/049757 published 2 Jun. 2005.
Typically low refractive index thin films are made utilising sol-gel processes or vacuum evaporation deposition techniques. The known processes which rely on sol-gel techniques for producing thin films are complex, multi-step processes which are costly, involve high temperature and/or pressure steps in fabrication, and/or require a surfactant for templating. The films produced by these sol-gel processes are typically easy to damage and/or liable to delamination.
The following prior art documents are but a few examples of methods used to produce low refractive index films made from silica.
A number of prior art processes based on known sol-gel technology were described in our earlier application. In addition to those previously discussed, reference may also be had to U.S. Pat. No. 4,652,467. This patent describes a process of depositing a thin film on a substrate from a non-gelled solution made from alcohol and various hydrolysable metal alkoxides, such as TEOS. The described process aims to control porosity and pore size but is quite complex. The process includes adjusting temperature, pH and standing time before chilling, curing and heating to form the film.
U.S. Pat. No. 5,698,266 is directed to a modification of the standard sol-gel method of forming an anti-reflective coating by mixing ethanol, tetraethoxysilane (TEOS) and ammonia and hydrolysing for up to 48 hours to form a colloidal silicon oxide suspension dispersed in an aliphatic alcohol. The silica sol is then filtered to obtain silica particles for deposition onto a substrate, prior to drying. The coated substrate is placed in an ammoniacal environment for up to twelve hours to form an anti-reflective coating. This final step is claimed to improve the binding between the silica particles, and so renders the film more robust.
One of the few prior art processes for producing silica coatings without directly following the conventional sol-gel process is described in U.S. Pat. No. 6,231,989 assigned to Dow Corning Corporation. The patent describes a process of forming a coating from a solution comprising a resin containing at least two Si—H groups in a solvent, such as methylisobutylketone. The solution is coated to a substrate with about 5% of the solvent remaining in the coating. An aqueous basic catalyst causes condensation of the Si—H groups. The solvent is evaporated to leave a porous coating. In summary, the method uses an ammonia vapour step to improve adhesion and mechanical strength of the resultant film, but relies in the main part upon conventional sol-gel methods to produce the coating at room temperature and pressure. The preparation of suitable starting materials containing two Si—H groups is difficult, which limits the usefulness of the technique.
None of the known techniques use simple processing steps to produce films with controlled morphology, such as porosity and pore size.